1. Field
The present invention relates to disk drives for computer systems. In particular, the present invention relates to a disk drive adjusting predictive caching based on the temperature of the voice coil motor.
2. Description of the Related Art
Disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk typically comprises a number of concentric data tracks each partitioned into a number of data sectors. Access operations are performed by seeking the head to a target data track, and performing a write/read operation on the data sectors within the data track.
In order to avoid the mechanical latency associated with seeking the head to a target track, prior art disk drives typically implement “predictive caching” wherein a number of data sectors proximate the data sectors of a read command are cached, such as data sectors preceding the read command (pre-read data sectors) and/or data sectors following the read command (post-read data sectors). In this manner, if a subsequent read command received from a host requests the data sectors that are already cached, the data sectors can be read immediately from the cache memory thereby avoiding the mechanical latency of the seek operation.
Prior art disk drives also typically implement a rotational position optimization (RPO) algorithm for selecting from a command queue the next access command to execute in an order that minimizes rotational latency. That is, the commands in the command queue are evaluated to determine the amount of seek latency required to seek the head to the target track, as well as the rotational latency for the head to reach the first target data sector. When predictive caching is enabled, and a read command selected from the command queue has additional rotational latency beyond that required to perform the seek, the additional rotational latency is used to cache additional data sectors (pre-read and/or post-read data sectors).
The prior art has also suggested to adjust the seek times to prevent the VCM from overheating, which may cause the material molded over the voice coil to delaminate, lose its rigidity, and/or outgas particles that may contaminate the disk. Overheating the VCM may also degrade the strength of the fixed magnets that interact with the magnetic flux generated by the voice coil. As the temperature of the VCM rises, and the seek times are lengthened, the RPO algorithm is modified accordingly. The prior art teaches to adjust the seek times so that the VCM temperature remains just under a predetermined threshold, that is, to minimize the seek time relative to temperature. However, operating the VCM near its temperature threshold may degrade performance since it may preclude the RPO algorithm from selecting the most optimal access command from the command queue (i.e., the nearest access command but for the VCM temperature being too high), or selecting a command from the command queue that requires an additional revolution of latency due to missing the first target data sector (because the VCM temperature is too high).
There is, therefore, a need to compensate for VCM temperature in a disk drive so as to improve overall performance.